1. Field of the Invention
The invention relates generally to the field of welding, and more particularly to welding brittle materials such as cast iron or stainless steel.
2. Description of Related Art
While many methods exist for joining metals together, welding is generally the most preferable because of the following reasons: 1) welding can be used with nearly all metals; 2) welding yields high-strength joints; and 3) welding avoids the galvanic-corrosion problems that can result from soldering or brazing. Metals that are not very reactive, such as steels, can be welded using a simple torch flame, often oxyacetylene, in air. More reactive metals require an electric arc in an inert atmosphere, such as argon, to prevent excessive oxidation.
In the welding process, adjacent regions of two or more discrete pieces of metal are locally heated to the point of fusion and then allowed to run together. Filler metal of similar composition is often added to the molten pool to bridge and unite the separate pieces when the melt cools.
Because welding involves the use of localized high temperatures, and because virtually all materials expand when heated, stress and/or distortion may appear in welded pieces as they cool. In relatively malleable metals, such as steel and wrought iron, this is not a problem since the metal is able to deform slightly and relieve the stress. Moreover, it is relatively simple to position the pieces before the main welding operation through small "tack welds" to minimize the overall distortion after welding.
Unfortunately, welding brittle metals poses a special problem. Examples of brittle metals include cast iron and stainless steel. In the nuclear industry, stainless steel can become highly embrittled through prolonged exposure to high levels of radiation. When such brittle metals cool after welding, the stress cannot be adequately relieved by deformation and cracking occurs instead. Attempts to repair the resulting cracks by further welding serve only to worsen the problem since the resulting stresses create new cracks or cause existing cracks to grow.
Traditionally, cast iron welders used a technique known as "peening" to minimize cracking from a weld. Using this technique, the welder simply taps the metal repeatedly with a hammer up and down the weld seam as it cools. Peening has been proven to be very effective in reducing cracking; however, the exact mechanism by which it works is far from clear.
Not wishing to be limited by this theory, one explanation for the success of peening is that the sharp acoustic waves launched into the metal provide the grain structure of the metal just enough extra energy to slip past each other and relieve stress. Both acoustic and thermal energy are in the form of phonons. However, the phonons resulting from acoustic energy are coherent, in phase, and travel in parallel. Conversely, the phonons resulting from thermal energy are incoherent and travel in random directions with random wavelengths. It is theorized that, due to their coherence, the phonons from peening become focused to relieve stress in the metal and therefore prevent cracking.
While traditional peening has been successful, its effectiveness is due in large part to the skill and intuition of the welder. Thus, for the technique to receive widespread acceptance, it must be refined to produce results that are both reliable and reproducible. Moreover, hazardous applications, such as those involving radioactivity, would require peening to be done remotely to minimize the welders' exposure to radiation.
Modern applications for peening have generally employed a vibrating member to impart the ultrasonic waves into the welded element. For example, U.S. Pat. Nos. 4,466,565 to Miyazima and 5,494,207 to Asanasavest both teach the use of vibrating members to assist in bonding wires on an integrated circuit board or chip. Similarly, U.S. Pat. Nos. 5,364,005 to Whelan et al. and 5,540,807 to Akiike et al. teach the use of a general purpose welding tool that incorporates a vibrating member to generate the ultrasonic waveforms. While these inventions are all useful for their intended purposes, they are not readily adaptable for use in hazardous environments and are generally geared toward micro-weld applications. Thus, there is room for improvement in the art.